round corners inverted of a plane
Probably the easiest way to do this is to manually cut it out. In edit mode, press K to bring up the knife tool. Cut along where you want and press Enter when you're done. You can then delete the face you want.

To get straight lines, press C while in Knife mode for constrained angles.

If you want a cleaner mesh with only quads, use the knife tool again and connect the vertices from where you cut to the edge of the plane. Repeat this with each vertex to create quads.